Motion information can be an important aspect of image generation. For example, in frame sequences of a video signal, the only difference between one frame and a next frame may be the relative motion of objects within the frames. Video compression may take advantage of this fact by encoding image data for only certain of the frames of the video signal (e.g., encoding image data every fourth frame). For the frames in between the encoded frames, only the information needed to generate the missing frames is stored. Part of the information needed to generate the missing frames is information about the motion of the objects within the frames.
Another example of where motion information can be important occurs when a frame rate of an input signal is inconsistent with a frame rate necessary for an output device. Video is commonly recorded at 24 Hz, 25 Hz, 30 Hz, 50 Hz, or 60 Hz. Commercially-available television displays, on the other hand, have picture rates up to and beyond 120 Hz. To interface recorded video with such a television display, the original frame sequence from the recorded video must be up-converted using, for example, a picture rate converter. The picture rate converter typically operates by interpolating image frames at time instances between the frames recorded by the lower-frequency source device.
In simple picture rate converters, an image is often repeated in the destination display until a next image arrives from the source device, which may result in blur and judder when motion occurs. Motion compensation circuits may be used in picture rate converters to reduce these unwanted effects and achieve a high performance conversion for moving sequences. Motion compensation operates by estimating where objects of an interpolated picture should be located, based on a direction and a speed of movement of those objects. The direction and speed values may then be expressed as motion vectors, which are used to “move” the objects to the estimated positions in a newly generated frame. In motion compensation circuits, the presence of periodic structures in a frame sequence can result in visual disruptions in interpolated images (e.g., temporal discontinuities or breaks in the interpolated images).